StencilI/StencilJ/StencilK should be handled as half integers

The following

```
$ u@i,j@ = ( f_0@i,j+1+1/2@ ) $
```

should generate something like

```
grid.cellAt( i, j, u ) = (grid.faceAt( i, j+1, Face::posY, f_0 ))
```

NOTE: we generate faceAt for half integers
